1. Field of the Invention
This invention is related to the field of computer systems and, more particularly, to data storage systems.
2. Description of the Related Art
A file system may be defined as a collection of files and file system metadata (e.g., directories and inodes) that, when set into a logical hierarchy, make up an organized, structured set of information. File systems organize and manage information stored in a computer system. File systems may support the organization of user data by providing and tracking organizational structures such as files, folders, and directories. A file system may interpret and access information stored in a variety of storage media, abstracting complexities associated with the tasks of locating, retrieving, and writing data to the storage media. File systems may be mounted from a local system or remote system. File system software may include the system or application-level software that may be used to create, manage, and access file systems.
File system metadata may be defined as information that file system software maintains on files stored in the file system. File system metadata may include, but is not limited to, definitions and descriptions of the data it references. File system metadata may include one or more of, but is not limited to, inodes, directories, mapping information in the form of indirect blocks, superblocks, etc. Generally, file system metadata for a file includes path information for the file as seen from the application side and corresponding file system location information (e.g. device:block number(s)). File system metadata may itself be stored on a logical or physical device within a file system.
Hierarchical Storage Management (HSM)
Hierarchical Storage Management (HSM) is a data storage solution that provides access to vast amounts of storage space while reducing the administrative and storage costs associated with data storage. HSM systems may move files along a hierarchy of storage devices that may be ranked in terms of cost per megabyte of storage, speed of storage and retrieval, and overall capacity limits. Files are migrated along the hierarchy to less expensive forms of storage based on rules that may be tied to the frequency of data access.
In HSM file systems, data access response time and storage costs typically determine the appropriate combination of storage devices used. A typical three-tier HSM architecture may include hard drives as primary storage, rewritable optical as secondary storage, and tape as tertiary storage. Alternatively, hard drives may be used for secondary storage, and WORM (Write Once, Read Many) optical may be used as tertiary storage.
Rather than making copies of files as in a backup system, HSM migrates files to other forms of storage, freeing hard disk space. Events such as crossing a storage threshold and/or reaching a certain file “age” may activate the migration process. As files are migrated off primary storage, HSM leaves stubs to the files on the hard drive(s). These stubs point to the location of the file on the alternative storage, and are used in automatic file retrieval and user access. The stub remains within the file system of the primary storage, but the file itself is migrated “offline” out of the file system onto the alternative or tertiary storage (e.g. tape).